In recent years, it is the latest trend that personal authentication with higher reliability (authenticating an equipment user to be the pre-registered authentic user) is required for use of an information system, information terminal equipment, such as a personal computer (PC) serving as part of an information system, and small-sized information equipment, such as a cellular phone and a PDA (Personal Digital Assistant). Meanwhile, personal authentication based on biometric information is highly reliable, and using a fingerprint as biometric information in particular offers high convenience.
In personal authentication using a fingerprint, the fingerprint (a pattern composed of a ridgeline that can be in contact with a fingerprinting surface of a fingerprint sensor and a valley line not in contact with a fingerprinting surface thereof) of a finger of an authenticated person is taken as image information (a fingerprint image), using a capacitive sensing fingerprint sensor or an optical fingerprint sensor. Feature information (e.g., a ridgeline shape and position information on a branch point and an end point) is extracted from this fingerprint image, and the extracted feature information is verified with pre-registered registered information (registered feature information) of the authenticated person. Based on a result of the verification, a determination on whether the authenticated person is the authentic person, i.e., personal authentication is carried out. A ridgeline pattern formed of the shape of a ridgeline and a feature point given by position information on an end point and a branch point of a ridgeline are mainly used as feature information. The ridgeline pattern is referred to as “feature quantity of level 1” and the feature point is referred to as “feature quantity of level 2”.
A fingerprint sensor incorporated in information terminal equipment, such as a PC, or in small-sized information equipment, such as a cellular phone and PDA, has recently been reduced in size. Such a small-sized fingerprint sensor has a sensor area smaller than a finger, thus is incapable of acquiring an image of the whole fingerprint simply through one time image-taking.
For this reason, a fingerprint sensor having a small area acquires an image of the whole fingerprint by synthesizing a plurality of partial images that are taken consecutively through the sensor surface of the fingerprint sensor. A sweep fingerprint sensor is a fingerprint sensor that is equivalent to such a fingerprint sensor (e.g., see Patent Document 1: Japanese Laid-Open Patent Publication No. 10-91769 and Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-208620). This sweep fingerprint sensor is sufficiently shorter than a finger and has a square fingerprinting surface of a small area (sensor surface/image-taking surface). The sweep fingerprint sensor is thus used in such a way that a plurality of partial images of a fingerprint are taken as the finger is moved relative to the fingerprinting surface or the fingerprinting surface (fingerprint sensor) is moved relative to the finger, and that the whole of a fingerprint image is restructured out of the plurality of partial images. Information of a feature point (a branch point and an end point of a ridgeline) is extracted and generated from such a restructured fingerprint image, and personal authentication is carried out based on the information. Moving the finger relative to the fingerprinting surface is referred to as “sweep or slide”. The same method of synthesizing a plurality of partial images to acquire an image of the whole fingerprint is also applied to a small-sized flat fingerprint sensor (see Nonpatent Literature 1: A. Jain and A. Ross, “Fingerprint Mosaicking”, in Proceeding of IEEE International Conference on Acoustics, Speech, and Signal Processing (ICASSP), 4, pp. 4064-4067, (Orland, Fla.), May 2002).
With respect to the above fingerprint authentication, Patent Document 3 (Japanese Laid-Open Patent Publication No. 2002-024831) discloses an invention that enables verifying a fingerprint image read by a cellular phone with a registered fingerprint image at sufficiently high authentication precision. Patent Document 4 (Japanese Laid-Open Patent Publication No. 2004-005619) discloses an invention that enables verifying with a registered fingerprint image at sufficiently high authentication precision even in a case where only the partial fingerprint image is acquirable. Patent Document 5 (Japanese Laid-Open Patent Publication No. 2001-344605) discloses an invention that the reproducibility of feature points extracted from each of a plurality of fingerprint images is estimated to sort out a feature point to be used.
For an improvement in capability of distinguishing a person from another person, a method using the outline of a fingerprint ridgeline and position information on a sweat gland pore has been studied. A feature quantity representing such outline and position information is referred to as a feature quantity of level 3 in comparison with the above feature quantities (see Nonpatent Literature 2: Anil Jain, Yi Chen and Meltem Demirkus, “Pores and Ridges: High-Resolution Fingerprint Matching Using Level 3 Features”, IEEE TRANSACTIONS ON PATTERN ANALYSIS AND MACHINE INTELLIGENCE, VOL. 29, NO. 1, pp. 15-27, JANUARY 2007).
An image processing method of generating a high-resolution image by synthesizing consecutive shots of a plurality of low-resolution images, which is generally referred to as super-resolution processing, has been suggested for application to a digital camera, etc (Patent Document 6: Japanese Laid-Open Patent Publication No. 2007-271690).
When a sweep fingerprint sensor is used for inputting a fingerprint image, operation of moving a finger and a sensor surface relative to each other is required at the time of inputting a fingerprint. Since acquiring the fingerprint depends on an operation method in this manner, an image of the whole fingerprint may not be acquired if the operation method by a user is not proper. In such a case, synthesizing a plurality of images does not produce the whole image but produces only the image of a partial area of the fingerprint (e.g., only a fingertip). When a flat fingerprint sensor is used for inputting a fingerprint image, for example, only the image of a partial area is produced if the way of pressing the user's finger is not proper to cause such a situation that the user's finger is not brought into contact with a sensor surface in a sufficient area. Even if a plurality of images are synthesized, therefore, only the image of a partial area of the fingerprint may be acquirable. In other words, using a small-sized fingerprint sensor may lead to a disadvantageous case where a sufficient capability of distinguishing a person from another person may not be acquired when the user's operation method is not proper.
In the method using the outline of a fingerprint ridgeline and position information on a sweat gland pore, a feature quantity of level 3 is acquired as described above. The method, therefore, requires a fingerprint sensor having a high-resolution (about 1000 dpi or higher), and incorporating such a fingerprint sensor into a small-sized information equipment is difficult because of problems of price and mounting size.
According to the super-resolution processing (Patent Document 6) carried out by a digital camera etc., an amount of positional shift between low-resolution images is detected at a precision of magnitude of smaller than one pixel, and the low-resolution images are synthesized in accordance with the detected amount of positional shift. On a fingerprint image, an overall positional shift occurs due to the extension/contraction of skin. Because of this, even if position matching is successful in a partial area between images, the same position matching is not successful in other areas. Hence the super-resolution processing used in a digital camera cannot be applied directly to the fingerprint image.
Patent Documents 1 to 6 and Nonpatent Literatures 1 and 2 present no disclosure or suggestion related to the above requirements and problems and no disclosure or suggestion about a configuration etc., that solve the problems.